1. Field of the Invention
The present invention relates to a method for producing a thermal-type infrared sensor, by using a metal film or a metal oxide film. Herein, it is to be noted throughout the instant specification that such a thermal-type sensor may be called a metal-film bolometer.
2. Description of the Related Art
As a bolometer material for thermal-type infrared sensors, it has been known to use a film of metals such as platinum and a film of metal oxides such as vanadium oxide.
For example, a bolometer-type infrared array sensor is proposed by Shie and Weng in Sensors and Actuators, A33 (1992) pp. 183, as shown in FIGS. 1A and 1B, and is produced by forming a space 13 within a Si substrate 10 operable as a heat sink, mounting beams 8 over the space 13, and arranging thermosensitive area 9 (in a suspending manner) around the beams 8. In the structure, the thermosensitive area 9 is arranged at a part thermally separated from the Si substrate. Herein, the thermosensitive area 9 is formed from a bolometer material, and additionally comprises a platinum electrode 11 operable also as a bias electrode and a NiCr infrared absorption film 12.
When vanadium oxide is used as the bolometer material in such a bolometer-type infrared sensor, the resistivity of vanadium oxide should inevitably be controlled. More specifically, because a great number of crystalline phases of the oxides of vanadium are present in the region of 4 to 5 atomic valences of vanadium, the resistivity of the bolometer material varies, depending on each crystalline phase. In order to control the resistivity thereof, proposal has been made by Jorgenson and Lee, Solar Energy Materials, 14(1986), p.205, about doping elements of metal impurities such as Nb, Ta, W, and Mo.
Among bolometer materials, platinum is advantageous in that platinum is hardly oxidized. However, platinum is poor in workability and, as a result, it rarely used in a silicon integrated circuit process. Therefore, recent interest has been directed to a metal material in place of platinum.
Instead of platinum, A. Tanaka et al have tried to use, as a bolometer material, Ti which has a small thermal conductivity and which is very often used in the Si--IC production process. However, it has been found out that Ti is chemically active and is easily oxidized within a process carried out after deposition of Ti and within a process of forming a protection film. This brings about a reduction of the temperature coefficient of resistance which serves as a significant factor of the bolometer material. As a result, a reduction of sensitivity is inevitable when Ti is used.
Herein, an amount of doped impurities in metal oxide films can not be controlled when the metal oxide films are formed by a film formation process, such as sol-gel process, a sputtering process, a chemical deposition process and the like. Therefore, the resistivity cannot be controlled satisfactorily by the above-mentioned processes. Additionally, a temperature coefficient of resistance becomes small with the increase of the doped amount.